This invention relates generally to the field of spectroscopy, and, more particularly, a technique which is capable during target detection of suppressing structural background radiation but allowing for the detection of target radiation.
Spectroscopy is concerned with the production, measurement and interpretation of electromagnetic spectra arising from either emission or absorption of radiant energy by various substances. Emission spectra are produced when radiant energy from matter, excited by various forms of energy is passed through a slit and subsequently separated into its various components or wavelengths either by refraction in a transparent prism or by diffraction from a ruled grating or in a crystalline solid. The production of absorption spectra involves use of similar devices except that the excitation source is replaced by a suitable radiant energy source. Spectroscopic measurements of wavelengths and intensities of radiative energy are made using instruments called spectroscopes, spectrographs, spectrometers, or spectrophotometers.
Interpretation of the spectra provides information concerning atomic and molecular energy levels, electronic configurations of atoms and ions, molecular geometries, and chemical bonds. Empirical correlations of the spectral characteristics with chemical and physical properties of matter provide a basis for qualitative and quantitative chemical analysis.
The form of spectroscopy which has been found to be the most superior is that of Fourier spectroscopy. There are many reasons for this superiority, but the two most often referred to are the multiplex advantage and the aperture advantage. In addition, absolute wave number accuracy is guaranteed by the known wavelength used for carriage control, the physical apparatus is simple and both stray light and overlapping spectral orders are eliminated.
A double-beaming spectroscopy technique which has proven to be the most effective in overcoming many of the problems associated with Fourier spectroscopy is set forth in patent application Ser. No. 662,995 filed Mar. 1, 1976 by one of the inventors of the instant application. In this technique two beams are fed onto opposite faces of a beamsplitter of an interferometer. The output which is common to both beams is suppressed in the output interferogram; resulting in a measurement of only the desired or wanted radiation.
However, even with this technique the presence of large unwanted background radiation presents a problem for detection, location, tracking, homing and obtaining spectral information of faint sources or targets. Such a situation occurs, for example, in astronomy (stars, etc.) and surveillance of military targets.